This is useful...
Koh Tao Weather: Diving Season - The Best Month to visit Koh Tao
I've only gone diving from Koh Tao in these months:
December 2009 - warm afternoon thunderstorms, but only for an hour or two
March 2011 - had a couple of days of rain overnight
August 2012 - sunny
May 2013 - sunny
September 2018 - sunny
For the most part, the waters around Koh Tao are pretty calm, which means diving everyday. Diving can be done on one side of the island or the other, depending on winds. It's very easy diving with little current. If you're out to hone your diving skills and get a lot of dives in, then this is the place, as it's quick to book dives and cheap, especially if you get a multi-day pass with a dive shop, which means up to five dives a day until the pass ends. Most of the time, it's four dives, with deeper dives in the morning and shallower dives in the afternoon, usually with OW classes. Night dives aren't as often.
What I like about Koh Tao are the easy days... 6:30 AM meet up at the dive shop, two morning dives, back to land for lunch, two afternoon dives, clean-up, rest a bit, dinner, a bit of night life, repeat.
The marine life around Koh Tao isn't as varied as what you've probably experienced. It took five trips to Koh Tao before I experience my first Whale Shark (and only, so far). There's a wreck dive at around 20+m, if you're looking to get that c-card. For deeper dives, there are occasional trips to Sail Rock or Chumpon Pinnacle, which are much longer rides out and adds fuel surcharges beyond the multi-day pass.
What did you find lacking at those other locales?
